1. Field of the Invention
The present invention relates to an auto refresh control circuit of a semiconductor memory device, more specifically, to an auto refresh control circuit for ceasing the operation of buffer during the auto refresh operation, thereby decreasing current consumption and minimizing power noise.
2. Description of the Related Art
Generally, a dynamic random access memory (DRAM) includes a capacitor as a unit memory device storing a data, which is called a cell. When a data of ‘1’ (or ‘high level’) is stored in the cell, a high potential is applied to the cell, and when a data of ‘0’ (or ‘low level’) is stored in the cell, a low potential is applied to the cell. In an ideal case, the capacitor constituting the cell maintains stored charges, so long as a potential of a connection terminal of the capacitor is not varied. However, actually, as the time elapses, the stored charges are lost as a leakage current. Accordingly, it is impossible to judge whether the recorded data is ‘1’ or ‘0’. In order to constantly maintain the data, a process of periodically sensing and amplifying the data stored in the cell, and restoring it in the cell is necessary. The process is called a refresh operation.
The refresh operation of the DRAM is divided according to a performing method. The RAS only refresh operation consists of a cycle of externally applying a row address strobe signal and a row address on which the refresh operation is performed, raising a word line selected by the row address, amplifying and re-writing the data of the whole cells connected to the word line by a sense amplifier, and dropping the word line. In this case, in order to refresh the whole cells of the DRAM, all the row addresses must be externally inputted.
So as to overcome the aforementioned disadvantage, there has been suggested the auto-refresh operation (or CBR refresh: CAS before RAS refresh). The auto-refresh operation carries out the ‘sensing-amplifying-restoring’ operation, identically to the RAS only refresh operation. However, since the row addresses are sequentially generated inside, it is not necessary to externally input the row addresses.
FIG. 1 is a block diagram illustrating a conventional auto-refresh control circuit of a semiconductor memory device.
The auto refresh control circuit includes a clock enable signal buffer 1, a power-down detector 2, a clock buffer 3, a command buffer 4, a command decoder 5, an address buffer 6, an address latch 7, an active control unit 8, an auto refresh delay unit 9 and a wordline enable unit 10.
The clock enable signal buffer 1 buffers a clock enable signal CKE. When the clock enable signal CKE is disabled to a low level, the operation of buffers ceases.
The power-down detector 2 receives the clock enable signal CKE from the clock enable signal buffer 1, and outputs an enable signal EN for controlling the operation of buffers according to the status of the clock enable signal CKE.
The clock buffer 3 outputs a clock pulse signal CLKP by buffering the clock signal CLK externally inputted. The clock pulse signal CLKP operates as a trigger signal for other input signals (command and address signals).
The command buffer 4 outputs a command signal COM by buffering external control signals CS, RAS, CAS, WE.
The command decoder 5 detects the state of the command signal COM outputted from the command buffer 4 in synchronization with the clock pulse signal CLKP outputted form the clock buffer 3, and then outputs the operation command according to the detected result. Here, the operation command is an auto refresh command AREF for example.
The address buffer 6 buffers an address signal ADD externally inputted.
The address latch 7 latches an address signal INADD outputted from the address buffer 6, and then outputs an address signal ADDLAT latched at a rising edge of the clock pulse signal CLKP outputted from the clock buffer 3.
The active control unit 8 enables an active signal ACT when a row active command and refresh command are inputted, and enables a precharge signal PCG when a precharge command is inputted. For example, if an auto refresh command AREF is inputted, an active signal ACT is generated.
The auto refresh delay unit 9 delays the active signal ACT outputted from the active control unit 8 as long as a refresh operation is performed, and outputs a delay signal DEL.
The active control unit 8 generates a precharge signal PCG in response to the delay signal DEL.
When the wordline control unit 10 receives the active signal ACT, the wordline WL is enabled. When the wordline control unit 10 receives the precharge signal PCG, the wordline WL is precharged.
FIG. 2 is a timing diagram showing an operation of the auto refresh control circuit illustrated in FIG. 1.
During the refresh operation, any external command is not noticed. After the refresh operation is finished, an operation of chip is performed according to an external command.
The enable signal EN maintains the state of a high level during an auto refresh operation. When a power down entry is inputted, the enable signal EN becomes at a low level.
While the enable signal EN maintains the state of low level, all buffers cease their operation.
Generally, the buffers do not consume current when the states of the external input signals vary, but constantly consume current while they operate.
Accordingly, a conventional auto refresh control circuit of a semiconductor memory device constantly enables input buffers during the auto refresh operation such that the current consumption increases.